mw.ForeignApi: don’t set origin for same-origin requests

mw.ForeignApi: don’t set origin for same-origin requests

If the foreign API has the same host as the current page (e. g. if it’s
actually the same wiki, or multiple wikis are installed under different
paths on the same host), then the browser will not send an Origin
header, and if we still set an origin parameter, then the API will
complain that the two don’t match. Detect this and unset the origin
parameter in that case.

Bug: T208601
Change-Id: Ia006f3dc3283ce3f81d4d72cbe9676a00797c4d0